Bond Dissociation Enthalpy of α-Tocopherol and Other Phenolic Antioxidants

Article abstract of DOI:10.1021/jo00096a061

The equilibrium constants, K₁, for the reaction between galvinoxyl and a series of phenolic antioxidants have been determined by means of EPR spectroscopy.With aroxyl radicals decaying at appreciable rates, K₁ was obtained by performing kinetic analyses of the time dependence of the concentrations of the equilibrating radicals after mixing the reactants.In two cases the temperature dependence of K₁ was also studied and the entropy change for the equilibration reaction was determined.Bond dissociation enthalpies, DH, of the ArO-H bond of the examined phenols were calculated by comparison with the known value of 2,4,6-tri-tert-butylphenol (81.24 kcal mol^-1).A larger than expected DH value was found for probucol (81.03 kcal mol^-1) and an explanation of this behavior was given in terms of the preferred conformation adopted by the para alkylthio group.The DH value of α-tocopherol (78.93 kcal mol^-1) was found to be very close to that of the phenolic precursor of galvinoxyl (78.80 kcal mol^-1) and somewhat larger than that of 2,6-di-tert-butyl-4-methoxyphenyl (77.61 kcal mol^-1).